Diesel engines, some gasoline fueled engines, and many other hydrocarbon fueled engines and power plants are operated at higher than stoichiometric air-to-fuel mass ratios for improved fuel economy. Many such engines comprise several cylinders, each with a reciprocating piston, into which air and fuel mixtures are sequentially introduced for combustion, and from which an exhaust gas stream is continuously expelled through an exhaust manifold into an exhaust conduit for eventual discharge into the ambient environment. Such engines that are controlled to operate at higher than their stoichiometric air-to-fuel mass ratio are sometimes called lean-burn engines, and the exhaust gas stream that they produce is called a lean exhaust, because it contains more oxygen from the excess air delivered to the cylinders of the engine.
Such lean-burning engines and other power sources produce a hot gaseous exhaust with relatively high contents of oxygen, water, and nitrogen oxides (NOx). In the case of diesel engines, the temperature of the exhaust gas is typically in the range of 50-150 degrees Celsius from a cold engine and 200-400 degrees Celsius from a warmed-up engine (depending, for example, on engine load), and has a representative composition, by volume, of about 10% oxygen, 6% carbon dioxide, 5% water, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance substantially nitrogen. The exhaust gas often contains some very small carbon-rich particles. And to the extent that the hydrocarbon fuel contains sulfur, the exhaust from the combustion source may also contain sulfur dioxide. It is desired to treat such exhaust gas compositions to minimize the discharge of any substance to the atmosphere other than nitrogen, carbon dioxide, and water. A representative value of the flow rate of such an exhaust stream, with respect to the effective volume of exhaust treatment devices, is, for example, 25,000 h−1.
The NOx gases, typically comprising varying mixtures of nitrogen oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream. It is found that when much of the NO is oxidized to NO2, there are selective catalytic reduction compositions and flow-through catalytic reactor designs for reducing much of the NO and NO2 in the hot exhaust to nitrogen before the exhaust is discharged from the exhaust system. So, in many exhaust treatment systems for lean burn engines a suitable flow-through oxidation catalyst body is located suitably close to the engine exhaust manifold to promote the effective and timely oxidation of NO and CO and HC in the exhaust. A second catalyst material is located downstream from the oxidation catalyst reactor in the flowing exhaust gas stream for the reduction of much of the NO and NO2 to nitrogen and water. Sometimes a reductant material is added to the exhaust gas to enable the selective reduction reaction, and other times the engine may be repeatedly, but very briefly, operated in a fuel-rich mode to supply small amounts of unburned fuel as a reductant for the nitrogen oxides.
On cold engine start-up, these oxidation and reduction catalyst materials must often be heated from an ambient temperature to their respective operating temperatures by the exhaust stream. It is necessary to convert most of the carbon monoxide and unburned hydrocarbons in the exhaust to carbon dioxide and water and to convert most of the NOx to nitrogen and water during all stages of engine operation, including the period when the exhaust system is being heated.
The upstream oxidation reactor with its catalyst material, close to the engine, is heated first by start of flow of the exhaust stream on engine start-up. But the downstream reduction catalyst material is farther from the heat source and slower to reach an operational temperature. During such a heating period some NOx material may pass untreated through the reduction catalyst material. The inventors herein recognize a need to make provision for improved handling of relatively cool NOx-containing exhaust gas until continued engine operation and exhaust gas flow can heat the reduction catalyst material to a temperature at which NOx can be effectively chemically reduced to nitrogen and water.